This invention relates to a satellite signal receiving apparatus that receives the vertically and horizontally polarized radiations sent from an artificial satellite.
Known communication satellites send out transmission radiation by alternately changing the plane of polarization of the transmission radiation for each adjacent channel, so that communication frequencies are effectively used. For example, JCSAT by Nihon Tsushin Eisel Co., Ltd., Super Bird by Uchu Tsusin Co., Ltd. and other communication satellites are available.
The satellite signal receiving apparatus for receiving radiation from such known communication satellites is disclosed in Laid-open Japan Patent Application No. 61-195094. The satellite signal receiving apparatus includes a receiving antenna located outdoors and provided with two converters and a switch. The two converters convert horizontally and vertically polarized radiation signals to the transmission signals having a specified frequency band, respectively. The switch selectively outputs either one of the transmission signals having the frequency converted by each converter. By controlling the switch at the side of the channel selecting unit located indoors, the desired polarized radiation signal is obtained at the side of the channel selecting unit.
In the aforementioned satellite signal receiving apparatus a signal line is provided for transmitting the switch driving signal from the channel selecting unit to the switch of the receiving antenna. A transmission line is also provided for coupling the receiving antenna and the channel selecting unit and for transmitting received signals. Therefore, when the satellite signal receiving apparatus is installed, the signal line and the transmission line have to be coupled separately. The installation work of the satellite signal receiving apparatus is thus complicated.
To solve such problem, the inventors of this invention developed the control over the switch without using the additional signal line for controlling the switch. Specifically, by providing a switch circuit at the channel selecting unit and a control circuit at the receiving antenna, the switch can be controlled without the signal line. The switch circuit switches the source voltage to be supplied to the receiving antenna into high or low source voltage, and the control circuit controls the switch according to the magnitude of the source voltage supplied from the channel selecting unit to the receiving antenna.
Generally in the satellite signal receiving apparatus, power source is supplied from the channel selecting unit to the receiving antenna via the transmission line for transmitting the received signals from the receiving antenna to the channel selecting unit. Therefore, when the plane of the polarization is switched and controlled by changing the magnitude of the source voltage, the receiving antenna and the channel selecting unit can be coupled only by the single transmission line. The installation work is thus simplified.
When the magnitude of the source voltage is changed, however, the constant-voltage circuit having input voltage with wide variations allowed is required for the receiving antenna, so that the converter can be operated with both the high and low source voltages. When the high source voltage is supplied to the receiving antenna, the input voltage to the constant-voltage circuit is dropped so much that the heat amount released from the constant-voltage circuit increases. Since raised temperature deteriorates the noise factor and other electric characteristics of the converter, the configuration of the converter has to be enlarged, or other actions against heat release are required.
Another problem is that the source voltage fed into the constant-voltage circuit decreases according to the length of the transmission line. The length of the transmission line varies with the position of the satellite signal receiving apparatus or other conditions. The greater the difference between the high and low source voltages is, the more precisely the difference between the source voltages can be determined regardless of the length of the transmission line, and the more precisely the switch can be switched and controlled. When the channel selecting unit outputs to the receiving antenna the high source voltage much higher than the low source voltage and the transmission line is short, however, the constant-voltage circuit has to receive extremely high source voltage. Consequently, when the polarized radiation signal is switched by changing the source voltage to be supplied to the receiving antenna, the size of the source voltage output from the channel selecting unit is difficult to set.